Influenza viruses are enveloped, single stranded negative-sense, segmented genome RNA viruses of the family Orthomyxoviridae. Influenza viruses are divided into three distinct types A, B and C; only types A and B have been identified as a concern in human pathogenicity.
Influenza A viruses are subtyped based upon antigenicity and genetics of their surface proteins, hemaglutinin (HA) and neuraminidase (NA), which are the major targets of the host organism's immune system. Contemporary circulating seasonal influenza A viruses are classified as H1N1 or H3N2. Influenza B viruses are mainly found in humans. All types of influenza have been shown to undergo antigenic shift and drift, though at different rates.
2009 influenza A/H1N1, also referred to as “swine flu” or 2009 influenza H1N1, is a new influenza virus that was first detected in humans in the United States and Mexico in April 2009. 2009 influenza A/H1N causes a range of influenza-like illness in humans, including cough, fever, sore throat, body aches and headache. The virus is highly contagious and spreads quickly.
Seasonal influenza strains (such as influenza A and influenza B) customarily peak in incidence and disease with a seasonal periodicity. Seasonal influenza currently circulating consists of influenza A/H1N1, influenza A/H3N2 and influenza B.
Certain strains of influenza are resistant to the antiviral drug oseltamivir (trade name Tamiflu®). For seasonal influenza A/H1N1, resistance is indicated by an RNA sequence mutation resulting in a CAT→TAT substitution that causes an amino acid change at position 275 (H275Y) in the neuraminidase protein. The change in the coding triplet from CAT to TAT is reflected by a change in the encoded amino acid. CAT normally encodes a histidine while the TAT coding triplet encodes a tyrosine. For 2009 influenza A/H1N1, resistance is also indicated by the same protein sequence mutation resulting in a histidine-to-tyrosine substitution at position 275 (H275Y) in the neuraminidase gene, but the RNA codon change is CAC→TAC.
In the United States, more than 200,000 people are hospitalized from influenza-related causes and an average of 36,000 people die from influenza-related complications annually. Transmission of the influenza virus occurs by aerosol, such as coughing and sneezing, and with contact with nasal discharge. Close contact and indoor environments favor transmission. Humans infected with seasonal and 2009 influenza A/H1N1 shed virus and may be able to infect others from 1 day before showing signs of illness to 5 to 7 days after becoming ill. The human influenza viruses are easily transmitted from human to human.
Symptoms of influenza A and B infections are characterized by fever, chills, anorexia, headache, myalgia, weakness, sneezing, rhinitis, sore throat and a nonproductive cough. In approximately half of all cases, nausea and vomiting may occur.
Traditional testing for influenza is performed using viral culture methods. Currently, the majority of influenza testing is performed using rapid lateral flow assays or rapid antigen detection assays, which are designed to either detect and discriminate influenza A and influenza B, or simply detect influenza A.
In the United States, four prescription antiviral medications, oseltamivir, zanamivir, amantadine and rimantadine, are approved for treatment and chemoprophylaxis of influenza. The majority of 2009 influenza A/H1N1 remains sensitive to oseltamivir and zanamivir but are resistant to amantadine and rimatadine, however, oseltamivir resistance has recently been identified in 2009 influenza A/H1N1 patient isolates. This, along with documented widespread oseltamivir resistance in seasonal influenza A/H1N1, presents challenges for the selection of antiviral medications for treatment and chemoprophylaxis of influenza.
Influenza detection and differentiation, in combination with identification of oseltamivir resistance in 2009 influenza A/H1N1 strains, would allow for improved treatments of viral infections. A rapid and accurate diagnostic test panel for the simultaneous detection and differentiation of influenza A, influenza B, and 2009 influenza A/H1N1 and the determination if the mutation for oseltamivir resistance is present in samples positive for 2009 A/H1N1, therefore, would provide clinicians with an effective tool for identifying patients symptomatic for one or more of the influenza viruses and subsequently supporting effective treatment regimens.